1. Field of Use
This invention relates generally to multiplexers and demultiplexers for optical signals. More particularly, this invention relates to multiplexers and demultiplexers that combine and divide light signals on the basis of their wavelength components and highly non-uniform holographic gratings used to disperse light waves.
2. Description of the Prior Art
The performance of an optical transmission depends on the capabilities of the materials used to build the system and the overall efficiency of the optical system. The overall efficiency of the system, of course, is dependent on the efficiency of the individual components of the system. First, insertion losses of components such as couplers and of more complex components such as optical multiplexers and demultiplexers (collectively referred to herein as multiplexers unless otherwise noted) can greatly limit the efficiency of an optical transmission system. The insertion loss of a particular optical component is approximately the sum of the insertion losses of the elements that make up the optical component. The insertion loss attendant state-of-the-art wavelength division multiplexers is high, thus decreasing the efficiency of an otherwise generally efficient optical system. The individual losses that make up the total insertion loss of a wavelength division multiplexer are represented by the following loss equation: l.sub.t =l.sub.d +l.sub.g +l.sub.a +l.sub.f where l.sub.d is the dispersion loss, l.sub.g is the grating reflection loss, l.sub.a is the aberration loss, and l.sub.f is the Fresnel reflection loss. In state-of-the-art multiplexers l.sub.t easily can greatly exceed 3 dB in losses.
Each of the above individual losses can be identified with certain components or characteristics of a wavelength division multiplexer. Dispersion-broadening losses, l.sub.d, are those losses due to chromatic broadening of the angular spectrum (beam spread) within the multiplexer. These losses are determined by the linewidth of the light source, wavelength separation between channels, and the relative positioning of the light sources and detectors about the optical axis of the multiplexer. Losses due to angular dispersion-broadening result because dispersion of the incoming light signal within the multiplexer causes the dispersed beam to have a larger spot size than the fiber core; thereby a portion of the light is not channeled into the output fiber. Grating losses, l.sub.g, are the result of imperfections in the dispersion grating. Aberration losses, l.sub.a, primarily are due to off-axial and chromatic aberration. Fresnel losses, l.sub.f, primarily are due to the light signal passing through the glass-air interface on both sides of the focusing lens. State-of-the-art wavelength division multiplexers have not achieved high efficiency. The losses in nearly all of these categories are high. As a consequence, the efficiency of the optical transmission system into which they are placed is drastically reduced.
Furthermore, state-of-the-art wavelength division multiplexers cannot handle multiple wavelength ranges over a broad spectrum. State-of-the-art multiplexers are limited by the physics of single surface relief metallic gratings. Such multiplexers have uniformly achieved only low-channel density, in the range of 1-4 channels. Similarly, such multiplexers have been severely limited in bandwidth, in the range of 10-20 nm. One multiplexer component that has limited the bandwidth of multiplexers has been the diffraction grating. State of the art surface relief gratings, and even Bragg holographic gratings and mirrors, have severely limited bandwidths. Given that the desired, usable bandwidth range is 0.8-1.3 .mu.m, about 500 nm in width, it can be seen that there is a great need for a broad band, high channel density multiplexer as well as one that is highly efficient.